Update the minor_rev property with client Method fails

Hi, We use a revision scheme based on Major Revision (B) and Minor Revision (01, 02, 03). I want to update the minor_rev property of ItemType Part with the following a JavaScript Method triggered from a Form button:

var updateItm = inn.getItemById("Part", "-------------id-------------");
updateItm.setAction("edit");
updateItm.setAttribute("doGetItem", "0");
updateItm.setProperty("major_rev", "B"); // <-works!
updateItm.setProperty("minor_rev", "02"); // <-don´t work!
updateItm = updateItm.apply();

The minor_rev property is a standard string and there should be nothing special with that property. I have all necessary permissions to perform this action. The corresponding Part is in state Preliminary and I am the Owner. The Method works fine for changing the major_rev, keyed_name, etc. . But even though I can easily override all kind of properties in my Part, the minor_rev property steadfastly refuses to change. It stays the same value as before. If nothing works, I will use a Server Method for this change. But I right have no single idea, why this Method don´t work. What could be wrong here? Is there anything special with the minor_rev property? For me it looks like all other properties. Thanks again! Angela